Plating Thickness Gauge Calibration Standards - Introduction to Calibration Foils and Substrates
1. Calibration foils
For magnetic methods, "foil" means a non-magnetic metallic or non-metallic foil or shim. For eddy current methods, plastic foils are usually used. The "foil" facilitates calibration on curved surfaces and is more suitable than a standard sheet with a cover layer.
2. Substrates
(a) For magnetic methods, the magnetic properties and surface roughness of the substrate metal of the standard should be similar to those of the substrate metal of the part to be tested. (b) For eddy current methods, the electrical properties of the base metal of the standard shall be similar to those of the base metal of the test piece to be tested. To confirm the suitability of the standard, the base metal of the standard may be compared with readings taken on the base metal of the test piece to be tested.
(b) If the thickness of the base metal of the piece to be tested does not exceed the critical thicknesses specified in Table I, calibration may be performed by either of the following two methods:
(1) Calibration on a metal standard piece of the same thickness as the metal substrate of the piece to be tested;
(2) Lining the metal standard or test piece with a metal of sufficient thickness and similar electrical properties, provided that there is no gap between the base metal and the lining metal. The liner method cannot be used for specimens with coverings on both sides.
(3) If the curvature of the cover layer to be tested has reached a point where it cannot be calibrated on a flat surface, the curvature of a standard piece with a cover layer is not acceptable.
The curvature of the standard sheet with the cover layer or the curvature of the base metal placed under the calibration foil shall be the same as the curvature of the specimen.
